The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Since Release 5 (R5) of the 3rd Generation Partnership Project (3GPP), the Universal Mobile Telecommunications System (UMTS) core network has been divided into three subsystems: a Circuit Switched (CS) domain, a Packet Switched (PS) domain and an IP Multimedia Subsystem (IMS).
The CS domain is used for providing users with connections of circuit switched service. The CS domain includes:
a Mobile Switching Center (MSC) (which may be further divided into MSC server and CS-Media Gateway Function (CS-MGW)) for performing switching and signaling control functions in circuit switched services;
a Gateway Mobile Switching Center (GMSC) which is an MSC used for mobile user routing and addressing in a network, which may be integrated into the MSC or be an independent device;
an InterWorking Function which is closely connected to the MSC and used for interworking between a Public Land Mobile Network (PLMN) and an Integrated Service Digital Network (ISDN), a Public Switched Telephone Network (PSTN) or a Public Data Network (PDN), and mainly for converting the signaling; the specific functions of IWFs vary according to different services and networks.
The PS domain is used for providing users with connections of packet switched service. The PS domain includes: a General Packet Radio Service (GPRS) support node (GSN) (including Serving GSN (SGSN) and Gateway GSN (GGSN)) and a Border Gateway (BG).
The SGSN and GGSN is used for transmitting data packages for packet switched service users; the service GSN (SGSN) provides a connection between a Core Network and a Radio Access System such as Base Station Subsystem (BSS) or Radio Network Subsystem (RNS), performs the functions of mobility management and session management in packet switched data services and manages the mobility and communication services of a Mobile Station (MS) in a mobile network; the GGSN acts as an interface between a mobile communication system and other PDNs and further performs the function of querying location information; both the SGSN and the GGSN may provide charging information.
The Border Gateway (BG) is used for interworking between two GPRS networks and ensuring the security of network interworking.
In addition, the common functional entities shared by the CS domain and the PS domain include:
a Home Location Register/Authentication Center (HLR/AuC). The HLR is used for managing subscription data and location information. The subscription data includes at least one of: a Mobile Station International ISDN Number (MSISDN), an International Mobile Subscriber Identity (IMSI), a Packet Data Protocol Address (PDP ADDRESS), subscribed telephony services, supplementary services and so on. The location information may include: a Mobile Switching Center/Visitor Location Register (MSC/VLR) number, an SGSN number, or a Gateway Mobile Location Center (GMLC) address. The AuC is used for storing the authentication algorithm and key of a user.
The common functional entities shared by both a CS domain and a PS domain also include a Visitor Location Register (VLR) for processing various data of the subscribers currently visited, an Equipment Identity Register (EIR) for storing the user equipment identities such as an International Mobile station Equipment Identity (IMEI), and a Short Message Service Gateway MSC (SMS-GMSC/SMS IMSC).
The IMS is a subsystem added in the 3GPP R5 on the basis of the existing PS domain. The IMS employs the PS domain as a bearer channel for the transmission of its upper level control signaling and media data, adopts a Session Initiation Protocol (SIP) as an IMS service control protocol, and provides abundant multimedia services for subscribers by separating the service control from the bearer control, and by utilizing the characteristics of the SIP, i.e., simple, extensible and convenient for a media combination. The main functional entities of the IMS include: a Call Session Control Function (CSCF) for the control of subscriber's registration and session control, an Application Server (AS) for various kinds of service logic control, a Home Subscriber Server (HSS) for the management of the subscriber's subscription data in a centralized manner, and a Media Gateway Control Function (MGCF) and an IMS Media Gateway (IM-MGW) for the interworking between the IMS and the CS domain. A subscriber may access the IMS through a proxy CSCF (P-CSCF) in the visited network, and then a session control, a service triggering control and a service control interaction with an AS are performed by the serving CSCF (S-CSCF) of the home network. The HSS in the IMS, whose functionalities include those of the HLR, is the superset of the HLR. However, due to a factor of networking process and so on, the HSS of IMS and the HLR of CS/PS domain may be deployed as entities independent of each other in the practical networking projects.
The IMS architecture defined by 3GPP solves all the crucial operability problems of multimedia service over IP, such as roaming charging, QoS and security. Therefore, the architecture and corresponding idea are recognized by the industry. Both 3GPP2 and TISPAN define the corresponding IP multimedia network architecture and service systems according to a 3GPP model and with reference to the 3GPP model. Meanwhile, 3GPP has started researches on the Interworking of WLAN access with 3GPP system (I-WLAN), Fixed Broadband IMS access (FBI), and the all-IP network (AIPN) which supports multiple access technologies. A subscriber may access the IMS according to the subscription of the subscriber through access networks of different access technologies with a single multi-mode terminal or terminals of various types, to obtain unified multimedia services, including VoIP services. Both the 3GPP, which is responsible for researches on the specifications of GSM system and WCDMA system, and the 3GPP2, which is responsible for researches on the specifications of CDMA system and CDMA 2000 system, have approved a Voice Call Continuity (VCC) work item doing researches on the service continuity between a CS domain call in a GSM, CDMA, WCDMA or CDMA2000 system and a VoIP service provided by accessing the IMS through a WLAN, and put forward solutions to the problems, such as the terminating network domain selection between CS domain and the IMS when a user acts as a called user, and the domain transfer between the CS domain and the IMS due to the move of the terminal, so as to ensure the service continuity and thus meet the demand of network and service development.
In the case that a user having subscribed to a VCC service acts as a called user, it is required in the VCC work item that the terminating network domain, the network domain used to deliver the incoming voice call, be selected by performing a synthetical determination based on network domain selection related factors so as to provide better service experience such as ensuring a higher success ratio of connecting the called user, and selecting a method with better quality or fewer fees. The network domain selection related factors include at least one of: user's registration status in the CS domain, user's registration status in the IMS, service and subscription data, network selection policy or preference set by an operator or the user, the case whether there is an ongoing call in one domain, and the capability of IP Connectivity Access Network (IPCAN) via which accessing to the IMS.
Moreover, with the development of the research of the VCC work item, a static anchoring and IMS centralized control solution for performing CS-IMS bidirectional domain transfer has been selected. The basic idea of the solution is that an Application Server (AS) is assigned to a user as a Call Continuity Control Function (CCCF) in the home IMS of the user, and all the control signaling of a CS domain call and an IMS session related to the user are anchored in the AS. The CCCF controls the call connection between a VCC user and the opposite user in a Third-party Call Control (3PCC) manner and implements a domain transfer as requested by the VCC user. As shown in FIG. 1A, a call connection between a VCC user and an opposite user is segmented into two segments in an AS. The VCC user establishes a new connection with the AS in a transfer-in domain when a domain transfer is required. After the new connection is established successfully, the AS performs re-negotiation on the connection between the AS and the opposite user so as to enable the opposite user and the VCC user to perform voice interaction through the new connection. The CCCF controls the replacement of the two segments of sessions with the VCC user respectively in the transfer-in domain and transfer-out domain during a domain transfer process.
Thus, an incoming call destined to a user having subscribed to a “VCC” service is routed to an IMS first to perform the above anchoring in order to control a possible domain transfer in the subsequent call processes, and then an Network Domain Selection function implemented in the IMS (IMS-NeDS) selects the domain for delivering the incoming call to a VCC terminal based on the above network domain selection related factors, i.e., directly delivering the incoming call in the IMS or delivering the incoming call to a CS domain as shown in FIG. 1B.
Although the work item of VCC aims at only voice services, besides VoIP services, great varieties of multimedia services may be provided for users in an IMS, in other words, the service capability of the IMS is far richer than that of the CS domain. Therefore, a service request received in the IMS and destined to a VCC subscriber such as an IMS session establishment request for performing multimedia interaction may be not appropriate to be directly delivered to a CS domain for connecting a called user. Specifically, in accordance with the prior art, the multimedia service may fall back, and even it is possible that the desired multimedia service cannot be provided, thus resulting in a service failure, if a service request is directly delivered to the CS domain without taking the category of the service request, e.g., the media components included in an incoming IMS multimedia telephony session, into consideration.
Moreover with the in-depth study of a terminating network domain selection function, it has been gradually recognized by the industry that the terminating network domain selection function is universally applicable in the process of delivering an incoming service destined to a called user with an ability of accessing multiple domains, in other words, the application scope of the terminating network domain selection function has gone beyond the VCC work item. Correspondingly, the terminating network domain that may be selected by the terminating network domain selection function is not limited to a CS domain and an IMS any more, but also includes different IP access networks via which the IMS is accessed, and a PSTN and an ISDN which are jointly called Circuit Switched (CS) networks with the CS domain and also have service capabilities much different from those of the IMS (For the sake of concise description, the PSTN and the ISDN, and the CS domain in the GSM, CDMA, WCDMA and CDMA2000 system are jointly called CS networks hereinafter). Therefore, the terminating network domain selection function as a general function should not be limited to a voice service yet, but great varieties of multimedia services in the IMS and the service capabilities of the IMS different from those of the CS network should be fully taken into consideration so as to avoid the following case: the provided multimedia service may fall back, or even a desired multimedia service cannot be provided, thereby resulting in a failure of a service, since a service request is directly delivered to the CS network without taking the service category of the service request into consideration.